docs: def _squeeze(system):
          """Squeeze out all singleton dimensions in the Subsystem.
  
          Reindexes the subsystem so that the nodes are ``0..n`` where ``n`` is
          the number of internal indices in the system.
          """
          assert system.node_indices == tpm_indices(system.tpm)
  
          internal_indices = tpm_indices(system.tpm)
  
          tpm = remove_singleton_dimensions(system.tpm)
  
          # The connectivity matrix is the network's connectivity matrix, with
          # cut applied, with all connections to/from external nodes severed,
          # shrunk to the size of the internal nodes.
          cm = system.cm[np.ix_(internal_indices, internal_indices)]
  
          state = utils.state_of(internal_indices, system.state)
  
          # Re-index the subsystem nodes with the external nodes removed
          node_indices = reindex(internal_indices)
          nodes = generate_nodes(tpm, cm, state, node_indices)
  
          # Re-calcuate the tpm based on the results of the cut
          tpm = rebuild_system_tpm(node.tpm_on for node in nodes)
  
          return SystemAttrs(tpm, cm, node_indices, state)